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//                      Didasko Tutorial Package
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// Create a Crs matrix corresponding to a 2D Laplacian problem
// on a cartesian mesh.

#include "Didasko_ConfigDefs.h"
#if defined(HAVE_DIDASKO_EPETRA)

#include "Epetra_ConfigDefs.h"
#ifdef HAVE_MPI
#include "mpi.h"
#include "Epetra_MpiComm.h"
#else
#include "Epetra_SerialComm.h"
#endif
#include "Epetra_Map.h"
#include "Epetra_Vector.h"
#include "Epetra_CrsMatrix.h"

// function declaration

void  get_neighbours( const int i, const int nx, const int ny,
    int & left, int & right,
    int & lower, int & upper);

// =========== //
// main driver //
// =========== //

int main(int argc, char *argv[])
{

#ifdef HAVE_MPI
  MPI_Init(&argc, &argv);
  Epetra_MpiComm Comm(MPI_COMM_WORLD);
#else
  Epetra_SerialComm Comm;
#endif

  // number of nodes in the x- and y-direction
  int nx = 5;
  int ny = 6;
  int NumGlobalElements = nx * ny;

  // create a linear map
  Epetra_Map Map(NumGlobalElements,0,Comm);

  // local number of rows
  int NumMyElements = Map.NumMyElements();
  // get update list
  int * MyGlobalElements = Map.MyGlobalElements( );

  // Create an integer vector NumNz that is used to build the Petra Matrix.
  // NumNz[i] is the Number of OFF-DIAGONAL term for the ith global equation
  // on this processor.
  // NOTE: NumNz can be specified to be an interfer, of value 5.
  // However, the procedure here reported is more general, and it is
  // representative of more complex situations, where the number of
  // nonzero per row can vary consistently.

  int * NumNz = new int[NumMyElements];

  double off_left  = -1.0;
  double off_right = -1.0;
  double off_lower = -1.0;
  double off_upper = -1.0;
  double diag      =  4.0;
  int left, right, lower, upper;

  for ( int i=0; i<NumMyElements; i++) {
    NumNz[i] = 1;
    get_neighbours( MyGlobalElements[i], nx, ny,
        left, right, lower, upper);
    if( left  != -1 ) ++NumNz[i];
    if( right != -1 ) ++NumNz[i];
    if( lower != -1 ) ++NumNz[i];
    if( upper != -1 ) ++NumNz[i];
  }

  // Create a Epetra_Matrix
  // create a CRS matrix

  Epetra_CrsMatrix A(Copy,Map,NumNz);

  // Add  rows one-at-a-time

  double Values[4];
  int Indices[4];

  for( int i=0 ; i<NumMyElements; ++i ) {
    int NumEntries=0;
    get_neighbours(  MyGlobalElements[i], nx, ny,
        left, right, lower, upper);
    if( left != -1 ) {
      Indices[NumEntries] = left;
      Values[NumEntries] = off_left;
      ++NumEntries;
    }
    if( right != -1 ) {
      Indices[NumEntries] = right;
      Values[NumEntries] = off_right;
      ++NumEntries;
    }
    if( lower != -1 ) {
      Indices[NumEntries] = lower;
      Values[NumEntries] = off_lower;
      ++NumEntries;
    }
    if( upper != -1 ) {
      Indices[NumEntries] = upper;
      Values[NumEntries] = off_upper;
      ++NumEntries;
    }
    // put the off-diagonal entries
    A.InsertGlobalValues(MyGlobalElements[i], NumEntries, Values, Indices);
    // Put in the diagonal entry
    A.InsertGlobalValues(MyGlobalElements[i], 1, &diag, MyGlobalElements+i);
  }
  cout <<  A;

#ifdef HAVE_MPI
  MPI_Finalize();
#endif

  delete[] NumNz;

  return(EXIT_SUCCESS);

}

void  get_neighbours( const int i, const int nx, const int ny,
    int & left, int & right,
    int & lower, int & upper)
{

  int ix, iy;
  ix = i%nx;
  iy = (i - ix)/nx;

  if( ix == 0 )
    left = -1;
  else
    left = i-1;
  if( ix == nx-1 )
    right = -1;
  else
    right = i+1;
  if( iy == 0 )
    lower = -1;
  else
    lower = i-nx;
  if( iy == ny-1 )
    upper = -1;
  else
    upper = i+nx;

  return;

}

#else

#include <stdlib.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
  puts("Please configure Didasko with:\n"
      "--enable-epetra");

  return 0;
}
#endif